In situ comparisons of ammonia volatilization from N fertilizers in Chinese loess soils

Ammonia volatilization loss from mineral N fertilizers was determined on a calcareous Chinese loess soil with a pH (CaCl2) of 7.7. An original in situ method that required no electricity or laboratory analyses was used. By means of a bellows pump, ambient air was drawn through four conical cups placed onto the soil (total area 400 cm(2)) and subsequently through an NH3-specific detector tube with direct colorimetric indication of the ammonia concentration (measuring range, 0.05-700 vol.-ppm NH3). Duration of measurement was about 3 min. Following N fertilization to winter wheat in 1990 and to summer maize in 1991, the application methods surface broadcast, uniform incorporation into the 0-15-cm layer, and for maize, a point placement at 10 cm depth were investigated. Ammonium bicarbonate and urea were applied at rates of 100 and 200 kg N ha(-1). In the autumn of 1990, ammonia losses following NH4HCO3 application were more than twice as large as with urea, fertilizer incorporation reduced NH3 losses 15-fold, and doubling the nitrogen application rate resulted in a 1.7-fold increase in the percentage of nitrogen loss. Cumulative ammonia fluxes were about 2 times higher in the summer of 1991. Comparing application methods in summer, losses were significantly (3 times) lower only with point placement. The above differences were all significant at the P<0.05 level. Due to the very low air exchange rate (0.9 volumes min(-1)), actual volatilization rates were underestimated by this method. Though not yielding absolute amounts, the Drager-Tube method proved very suitable for comparing relative differences in ammonia fluxes. The measurements clearly reflected the characteristic flux patterns for the different treatments and the effects of environmental factors on their time course.